Blackberry Gum

I have grown a few blackberrys in the past, different breeders to yours. I have done photos and autos and I found them to be quite picky on nutrients. Most the plants looked like yours at that stage. Only a few plants looked different. Could be due to the genetics. Unless your seeing shiny leaves, brown spots or anything else I wouldn't worry. One thing I did find with blackberry is they really got nitrogen build up quite quickly. Try increasing your P and K a bit and see if that has any positive development.